Telecommunication system employing voice frequency current for selective signaling



M. DEN HERTOG ET AL 2, 2 4 I TELECOMMUNICATION SYSTEM EMPLOYING- VOICE FREQUENCY Ju ly 1,1947.

Filed May 8, 1943 CURRENT FOR SELECTIVE SIGNALING M. DEN HERTOG ET AL 2,423,084 TELECOMMUNICATION SYSTEM EMPLOYING VOICE FREQUENCY CURRENT FOR SELECTIVE SIGNALING July 1, 1947.

2 Sheets-Sheet 2 Filed May 8, 1945 INVENTORS Ill/71970706 0f #67? 706 ESMMD R6. W/P/Gl/T BY AGENT Patented July 1, 1947 TELECOMlWUNICATION SYSTEM .EMPLOY- ING VOICE FREQUENCY CURRENT FOR SELECTIVE SIGNALING Martinus den Hertog, Antwerp, Belgium, and Esmond P. G. Wright, London, England, assignors to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application May 8, 1943, Serial No. 486,274 In Great Britain June 11, 1940 3 Claims. I The present invention relates to alternating current communication systems, (e. g. telephone systems) employing voice frequency current for selective signaling (which will be referred to herein as dialing) and supervisory signaling.

An object of the invention is to provide means for switching the forward alternating current signalingcircuit over from the dialing condition to the communication condition.

According to the invention the forward circuit is switched over from the dialing condition to the communication condition in response to a voice frequency signal transmitted back from a subsequent stage in the connection.

Embodiments of the invention will now be described by way of example, reference being made to the accompanying drawings comprising Figs. 1, 2, 2a and 2b.

Fig. 1 is a schematic diagram showing the layout of part of an alternating current communication system (e. g. telephone system) in which connections are set up over machine switching apparatus.

Figs. 2 and 2a in conjunction constitute a circuit diagram of apparatus employed at the incoming end of a toll line.

Figs. 2 and 2b in conjunction constitute an alternative circuit diagram of apparatus employed at the incoming end of a toll line.

Fig. 1 shows diagrammatically a known layout of a telecommunication (e. g. telephone) exchange network employing the Rotary system of automatic switching. This network comprises three exchanges X, Y and Z. Exchanges X and Y are connected by toll lines TLI terminated at their ends in toll line circuits TCI and T02 whilst exchanges Y and Z are connectedby toll lines 'ILlterminated at their ends in toll line circuits T03 and T04.

The apparatus in exchange X includes a finder Fl linked to a group selector GSI over a connecting circuit CCI which is associated with a register RGI over non-numerical switches SI. Similar apparatus is provided in exchanges Y and Z, the general operation of these registers and switches being well-known.

It will first be assumed that a connection is to set up from the subscriber's station A connected to exchange X to the subscribers station B connected to the exchange Y. The subscriber at A lifts his receiver and on receiving dial tone he dials the number of the station B. The code and numerical digitsdialed by the calling subscriber at A are all registered in the register RGI which retransmits them as and. Whenrequired for extending the connection over the appropriate numerical switches in succession. The code dig its stored by the register RG1 are translated to provide the routing digit (or digits) necessary to control the selection (or selections) to reach the required terminating exchange, whilst the numerical digits necessary to select the desired line at the terminating exchange are transmitted by the register RGI unchanged. When the selector GS! has been set in accordance with the first routing digit from the register RGI it proceeds to hunt over the toll lines TLI for an idle line.

The register RGI then transmits the second routing digit which is received by the incoming toll line circuit TC2.

The circuit TC2 upon recognizing that the call is to terminate at the exchange Y, causes the register RG2 to be connected up over the switches F2, S2. The register RG2 sends back to the register RG! 2. proceed to dial signal whereupon the register RGI transmits all the numerical stored digits to the register RG2. When the register RG2 has received all the digits it sends back over the line TLI an end of selection signal. The register RGI responds to this signal and thereupon releases itself and causes the speech circuit at the exchange to be. switched through. The register RG2 then proceeds to control the setting of the group selectors G82, G83 and the final selector F8! to connect up the line leading to the substation B. When the called subscriber answers, an answer signal is sent from exchange Y to exchange X to effect a metering or like operation.

It will nowlbe assumed that it is desired to extend a connection from substation A via the tandem exchange Y- to a substation 0 connected to the exchange Z. The operations which take place are the same as those already described.

up to the selection of an idle toll line TLI terminating in a toll line circuit TCZ. The second routing digit, however, in this case is difierent and causes the circuit T02 to start up a finder F3 in place of the finder F2 so as tocause the circuit TCZ to be connected over the finder F3 and the outgoing .tollline circuitv T03 to anidle toll line TLZ terminating in an incoming toll line circuit T0 at exchangeZ. The circuit T04 then sends.

back a proceed to dial signal which is received by the register RGI and causes it to send thenext routing digit, which is received by the circuit TC'4.

The circuit T04 upon recognizing that the call is to terminate atthe exchange Z causes the register RG3 to be connected up over the switches F4, S3. The register TG3 sends back a proceed to dial signal which is repeated at the tandem exchangesY. and.received by theregister. RGI. The register RGl fthereupon transmits all the: numerical stored digits to the register RG3. When the register RG3 has received all th nu-.- merical digits it sends back an end of selection signal. The register RGI respondsto th'i-ssignal and thereupon releases itself and-"causes the? speech circuit at the exchange X to be switched through. The register RG3 then DIOCGBdSFtO con-- trol the setting of the groupzselctors1GS4gGS5 and the final selector F82 toconnect-up the li ne. leading to the substation C. If the-callis to :be

routed through the exchang Z to a further exchange, the finder F5 is set in-rmoti'oniirrnplace' of the finder F4 and the call is routed? over the outgoing toll line circuit T05.

In systems having switching layouts similartothat. of Fig. 1, it has been found possible and consequently convenient, tomake the proceed to'-dial signal, the end' ofselection signal and the answer "signal the same; in the particular examplechosenthey all consist of a short interruption of the backward'loop:

It "has "been possible hitherto to make all these signalsrthe same forthe reason that the equipment at the originatingexchange has been able todiscriminate'betweenthem because of'the differentconditions existing at'the originating exchange when they are 1 received." The register RGI-hasbcenabldto recognize by its own conditionwhether'the-signal represents a proceed to dial signal or'the end of selection signal; and the, answer signal is not received'bythe registerRGl but by the connecting circuit CC I It is necessary toensurethat the end of selection""signal is received by the register RGI withir'i'a' predetermined fixed'period of time from the transmissiOnofthe last digit, because otherwise there may. be aodanger' ofthe signal being lost orrinterferred'with dueto the acti'onofthe call ing; subscriber; e': g; due 'to'the dialing of an extradigit: The; impulses hitherto employed in systems having switching layoutssimilar to that ofFigi. 1" havingbeen D. C." impulses or"'50-cycle- A..C;im'pulses0r a-combination thereof; e. the toll li'ne'TL'Lmaybe operatedon D'. C. impulses andftheitoll line""ILZ may be operated with 50 cycle A. Cf impulses;

Various difli'culties" arise when it is desired to employ impulses of alternating-current having" a frequency in the voice frequencyrange; and to transmit these forward-over the toll'line TIII or T132. for dialing; andbackwards for supervisory signaling. The tolllinecircuits'TC'l, TCZZ TG3; TClPare" in this'case modifidto convert the selective direct current impulses into voice' irequency-impulsesat the outgoingend ofthetoll lineand" vice versa at the I incoming end of 'th'e toli"line. It is necessary; however; that ir'nmedi atelyafter'all'the'voice frequency dialing ii'npulses fhQiVG been transmitted over TL] 'and" T132 theficircuits -TC2 and-TC'M'must be rendered ina sensitive? to"th e'- voice frequency signals in order to? protect the circuitfrom interference due to speech," line noises; room" noises; or due to theaction' of"- echo suppressors or other' apparatus associatedwith 'the'circuit.

It Will'be'; appreciated that the changes made to the" circuits TCI, T02; T03, T04 should'be as small as possibleand that they shall not-require anyichangesin othenportionsof the equipment. The; registerRGl will still-becapable-- of dis tinguishingbetween' proceedto'di'a "and nd of selection signals as before but it is not possible to communicate this information from the register. RGI to-the circuitJITCLwithout making extensive. changesiin the: existing register RGI and providing a path for sending appropriate signals from the register RGI to the circuit TCi. Unless modifications are made to the supervisory signals; thereforeythe circuit TCI will receive anfindefinitenumberof proceed to dial signals, followed by an"endof selection signal, followed by2arrr'answer signal It is only to the end of selection that theicircuit TCI has to respond, but -'it hasn'o means'for telling when this signal is received"unlessiaccording to the invention it is distinguishedin some way from the proceed to dial signals.-

A's'hastalready been pointed out, it is necessary that the end of selection signal should reach the originating register RGI without delay and it is, therefore, undesirable that the proceed to dial 'and end "of- "selectio'n signals which. reach the-incoming circuit T02 should be" subjected to an examination before -re=-transmission for the purpose o-f determining-WhiCh is-the end of "se lection-"signa'l. Itis; therefore; arranged ac cording to'th'e 'presentinventionthatthe end of selection signal A will be transmitted" back from TC2 in' precisely the'sam'e form as the" proceed to dial signals but that subsequent discrimination made by' the circuit' TC'Ziwill causethe transmission" of a special signal immediately following Theoutgoingcir'-- the'-proceed 'to'dial 'signal. cuit TCI will recognize this special signal "and'in response thereto" it" will cause the forward ir'npulsing' circuit to be" disconnected and Y the forwardspeeclr circuit to be connected'in its place. one method of voicefrequency signalingnow in' use utilizes currents of two voice frequencies (er-g; 600 cycles and 750 cycle's) connected 'to the line simultaneously'for'use'as a prefik signal whichwill' prepare thereceiver for'the' reception of 'a short impulse-of 'asingle frequency: These prefixed signals are particularly necessary after speech has" commenced "with'th'e consequent li'kelihood of interfering voice'frequency"currents: Before-'tlie* speech period, single frequencies-may'b'eused" without dangerfandit is proposed in'accordance with"the-presentinventionthat the proceed' to" dial signals shall each be transmitted *over' the tolllines TIi'l, T132, as'an' impulse'ofa single'frequency-"(e g: 750 cycles); The 'endbf-selection signal is" subsequently transmitted as an impulse of the= same single frequency; followed-"by the aforesaid"'sp ecial signal consisting'jof two voice frequencies-(e1g:600 cycls=and"750'cycles) 'sir'nultan'ecu'slyi Suitablearrangements for 'tiansmit ting these signalswill now be described wit'lr'reference-to-Fi'gsi 21 2z and 2bi The firstarrange'-- ment employs the circuit formed by Figs: 2"and 2a taken'in combination whils'tthesecond "arrangementemploys'tliecircuit' formed by Figs: Zan-d 2b uremia combination;

'lhla'first arrangement (Figs; 2' and 2d) is based by the"b'aclwarcl transmission ofa' proceed to' remair'rir'igdigits are transmitted'to the'terminatin'g"register "withbut 'interveningselection' processes and accordingly wilihout"-intervening' "pro"- ceed' to-dial signals. It follows that if the incoming terminating toll li ne circuit Tc i record's the-fatttliat two'digits passintherorwarwdirec:-

tion without an intervening proceed to dialf signal, then the next signal in the backward direction will be an end of selection signal. On the other hand, the switching arrangements may be such that certain selections to operators or special service positions are carried out by the transmission of only one numerical selection and, in consequence, the circuit ICE is powerless to make the necessary discrimination. It may be possible to avoid this difiiculty by modifying the switching layout to ensure that there are at least two numerical selections for every type of connection.

If this is not possible or desirable, use may be made of the second arrangement comprising Figs. 2 and 2b, to be described later.

Referring now to F gs. 2 and 2a, voice frequency impulses received over the toll line TLI are repeated over the outgoing speaking leads as direct current impulses.

For the sake of simplicity a relay A is shown to respond to the voice frequency impulses received over TLI, but it should be understood that these impulses are preferably received by a thermionic valve circuit, a relay similar to the relay A being operated from the output of the valve circuit. When the line TLI is taken into use, relay A is operated by a pulse of current over the line and causes the obvious operation of the relay B which locks up over contact bl until the connection is released. The contact 126 closes the forward loop circuit and the relay G operates when this loop circuit is complete, closing at Gl a circuit for the relay D over contacts 122, i2 and gl. The contact dl closes the obvious circuit for the relay E which looks over contact e I. Contacts 112 and (13 change over the circuit of relay A whilst contacts e2 and c3 prepare a circuit for the transmission of backward impulses. The first series of forward impulses transmitted over the line TLI causes corresponding intermittent operation of the relay A, and the contact a2 repeats the impulses over the outgoing D. C. circuit, the short circuit at ca having been removed. The contacts (13 maintain the relay G operated during the impulsing process.

' Relay A also causes the operation of the relay C through front contacts a4 and the back contacts lid. The operation of the relay C closes the obvious circuit for relay H, which looks. Relay C has, however, provided a locking circuit for itself at contact 02 and, being slow to release, remains operated throughout the digit. Subsequently relay B releases, but relay H remains operated, being held through contacts d4 and I25.

When the consequent selection has been completed a proceed to dia signal, consisting of a momentary opening of the backward loop causes the momentary release of the relay G. Relay G opens the circuit of relay D which releases and at contacts d2, d3 connects a voice frequency (750 cycles) impulse .to the line TLI. The release of relay D also causes the release of relay H. At the end of the proceed to dial signal, relay G re-operates and relay D also re-operates. The next train of impulses causes the re-operation of the relays C and H and the subsequent release of C at the end of the train. If this next train of impulses is immediately followed by a backward signal and the consequent release of relay G, the circuit TCZ will recognize this backward signal as another proceed to dial signal and the signal will be repeated back in the same manner as before. If, however, this next train of impulses is not followed by the release of relay G but is followed by a further train of impulses,

the circuit TCZ recognizes that routing selection has been completed and that the next backward signal will be the end of selection signal. When two successive trains of impulses are received without an intervening release of relay G, the first impulse of the second of these trains will close a circuit through contacts a4, 02 and k3 for relay J which is provided with a locking contact it. When the next backward signal (which must be the "end of selection signal) is received relay G releases D which immediately connects a 750 cycle impulse back over the line TLI.

It will be noted that in these circumstances, the re-operation of relay G at the end of the signal cannot cause the re-operation of relay D because the circuit of relay D is now held open atcontact '2. Due to the release of relay D, re

lay H releases and connects a second frequency (600 cycles) to the line TLI over contacts hl, M and contacts hl, M and contacts 7'3, 1'4. Relay H also opens the circuit of relay J but this latter relay, being slow to release, remains operated for a period during which the two frequencies are sent back over the line to constitute the aforesaid special signal. When relay J releases, the circuit is reclosecl for the relay D which re-operates. When the answer signal is received relay G releases momentarily whereupon relay D releases momentarily and connects a momentary pulse of voice frequency current (750 cycles) back over the line TLI.

Figs. 2 and 2b in combination show an alternative arrangement for use when it is found impossible to obtain a minimum of two numerical selections. This alternative arrangement contemplates a modification to the terminating register RG3 so that it will transmit two backward impulses for the "end of selection signal the second following the first with a relatively small delay. If such impulses are suitably spaced, they will pass through all the D. C. and 50-cycle links in a built-up connection because they may be considered as two closely spaced proceed to dia signals.

The operation of relay G on seizure causes the operation of relays D and E as already explained. The receipt of a proceed to dia signal (a single interruption of the backward loop) causes the momentary release of relay G, and the conse quent operation of relay C over contacts 92 and e5 and the release of relay D which transmits a voice frequency impulse (750 cycles) over the line TL. On re-operating, relay G closes a circuit for the relay H over contacts g2, cl and f5. Relay C is slow to release and relay H is maintained operated for the release period of relay C. In the case of an end of selection signal (a double interruption of the backward loop) the first interruption causes a voice frequency impulse (750 cycles) to be transmitted as just described. The second interruption causes relay G to release a second time before relay H is restored; relay J operates and looks. The operation of the relay J causes the release of relay H, and the connection of a second frequency (600 cycles) to the line TLI, as already described. Relay H at contact n3 opens the circuit of the relay C which, on release, opens the circuit of relay J; the slowrelease time of relay J controlling the duration of the impulse of current of the two frequencies simultaneously. On the release of relay J, the

relay D re-operates in preparation for the trans- 7 case: of. the first proceed to dial signal. subsequent proceed to-dial signal is sent back from-the registeriRGeand causes the circuit T64 to-operate'in:asimilar manner to the circuit 'lCZ and transmit backover the line TLziavoice frequenc impulse whicnpasses through the circuitsat'thewexchange Y and over the'line 'I'Ll to exchange X. The endof selection signal from the: register RG3 is converted into a voice frequency-signal in the circuit T64 and transmitted back to-exchange X-in thisform as will be-readily understood from the preceding description.

What isclaimed is: 4

' .'1-.-In an alternating current communication system, a plurality of exchanges, a plurality of lines'terminating at saidexchanges; means includingtrunk lines and a series-of stagesofselectors 'at said: e-xchangestfor first selecting trunks and selectors to buildup a connection between a calling. line at one exchange and a called line at a second exchange and then establish communication over said connection, a currentsource, means for sending from said source voice frequency selective and supervisory signals from one: exchange to another for controlling the building up of the connection, including supervisory' signal consistingof asingle-direct current impulse and another kind consisting ofa double direct: current impulse, means at one selector stage responsive to a voice frequency signal, and means forv transmitting back-the last-mentioned signal from a subsequent selector stage in the connection for establishing the communication over the connection.

-2. In= an alternating current communication systems plurality of exchanges, a plurality of linesterminating at said exchanges, means including; trunk lines and a series of stages of selectors at said exchanges for first selecting trunks and selectors to build up a. connection between a calling line at one exchange and a called ilinehat a secondexchange and then establish communication over said connection, acurrentsQurce, means for sending fromsaid source voice frequency selective and supervisory signals from .one. exchange to'another for controlling the building upotthaconnection, means responsive tooneikind of. supervisory signalto transmit back af single voice frequency and to another kind. of

The

supervisory signal to transmit back two voice frequencieameans-at one selector stage responsive to a voice frequency signal .and means for transmitting back thevlast mentioned signal from.

a subsequentselector stageinithe connection for establishing the communication over the connection.

3. In an alternating current communication system, a plurality of exchanges, a plurality of lines terminating at said exchanges, meansi'neluding, trunk lines and a series of stages of selectors atsaid exchanges for first selecting trunks and selectors to build up a connection between a calling line at one exchange and a called line at a secondexchange and then establish communication over said connection, acurrent source, means for sending from said source voice frequency selective and supervisory signals from one exchange to another for controlling the building up of the connection, means responsive to onekind of supervisory signalto transmit back a single voice frequency and to another kind of supervisory signal to transmit back two voice frequencies, means for changing. the voice frequency signal transmitted back upon the receipt at said subsequent stage of two successive. trains of selectivesignals withoutan intervening supervisory signal, means at one selector stage responsive to a voice frequency signal and means for transmitting back the lastv mentioned signal from a subsequentv selector stage in the connection for establishing the communication over the connection.

I MARTINUS DEN I-LERZI'OG. E'SMOND P. G. WRIGHT.

REFERENCES CITED The following references are of recordiin' the file of this patent:

OTHER REFERENCES Ex parte Quayle 1935, C. D. 11. 

